During the process of manufacturing a display panel constituting a display device, a display panel substrate provided with semiconductor elements, such as TFTs (Thin Film Transistors), may be used. Fine patterns that constitute the various electrodes and the like of the semiconductor elements are formed by patterning a metal film and the like on the substrate, using photolithography process which involves exposure and development with the use of a photomask having the patterns. For example, Patent Document 1 indicated below discloses a method of manufacturing an organic EL display device. The method includes forming organic EL elements arranged in a matrix on a substrate by photolithography process using an exposure mask having patterns (photomask).